1. Field of Invention
This invention relates to hydrofoils, and to sailboards, specifically to those equipped with hydrofoils which are capable of lifting the board clear of the water surface.
2. Description of Prior Art
Hydrofoils are appended to sailboards for the purpose of increasing speed or improving handling characteristics, or both. Higher speed comes essentially for free, since submerged hydrofoils can easily provide adequate lift while operating at much lower drag than planing hulls. The problem in the design of hydrofoil sailboards is that of providing rapid automatic corrective response to a number of destabilizing hydrodynamic effects, so that the sailor is able to control the craft. Automatic height control in waves is particularly important.
Providing this automatic response has proven so difficult, that despite widespread speculation among board sailors about hydrofoil sailboards, only a few patents have been issued and there is currently no example on the market. The typical board sailor has never seen hydrofoils actually attached to a sailboard.
I have found four prior designs for sailboard hydrofoils: two are disclosed in U.S. Pat. Nos. 4,508,046 (1985) to Coulter et al. and 4,715,304 (1987) to Steinberg, one is disclosed in German patent 3,130,554 Al (1983) to Jankowski, and one was manufactured by the Harken Company without patent. These designs cover the range of known hydrofoil configurations. Each has significant drawbacks which I shall discuss presently.
In contrast to the sparse hydrofoil sailboard prior art, that for other (larger) types of hydrofoil craft is extensive. Basic engineering is well covered; such topics as attitude stability, altitude determination, lateral force balance, and turning, at least in the absence of important surface effects, are completely developed. The subject of foil ventilation has been heavily researched. Much of this knowledge can be applied to hydrofoil sailboards.
However, the most vexing questions for the hydrofoil sailboard designer are not addressed in the art of other craft, as they derive expressly from two features of hydrofoil sailboards which distinguish such sailboards from larger hydrofoil craft.
First, by the nature of the sport, control must be obtained by simple operator movements, and preferably by mere weight shifts and sail position manipulations. Thus, much of the panoply of modern techniques for controlling larger hydrofoils, which involves active, powered, often computed, sensing and feedback to drive hydrodynamic flaps or pneumatic valves, is not available to the hydrofoil sailboard designer. In particular, hydrofoil sailboard stabilization must come readily from the interaction of hydrodynamic forces, operator and board mass forces, and board and foil configuration.
Second, there is the fact that sailboards are so small. Consequently, foils for them must necessarily operate very close to the water surface. In waves, proximity between the foils and the surface implies that sailboard hydrofoils penetrate the water surface as a matter of course, and even in flat water, such proximity leads to instabilities that do not exist for foils operating more deeply submerged. For instance, ventilation is a severe problem even for fully submerged hydrofoil configurations--a problem to which larger craft are resistant. Broaching--the movement of a foil through the water surface from below--is another problem. Broaching has severe hydrodynamical implications, and graceful recovery after broaching is crucial to successful design. The specific difficulty for small hydrofoil craft derives from the fact that a foil, having just broached and resubmerged, often carries down with it an air bubble stuck to its top surface. This bubble seriously reduces the lift produced by the foil compared to the lift produced at the same depth in the absence of the bubble. Typically, the result of this loss of lift is a radical sinking of the affected foil, which continues until either the bubble sheds spontaneously or the board itself hits the water. I call this phenomenon plunging.
All control problems in hydrofoil sailboards, and especially that of height maintenance, are exacerbated by the need for very quick correction, before the board falls the small distance from its normal operating height to the wave crests.
I now turn to specific hydrofoil art that relates to features of the present invention. I begin with knowledge related to the ability of a foil to track the water surface. Such tracking is useful in itself, but is more often used to control one or more other foils.
It has long been known that proximity to the water surface modifies the behavior of a fully submerged hydrofoil. To a first approximation, it loses lift as it approaches the surface. This effect is significant at depths of less than a chord length or so. It can be used in suitable circumstances to stabilize foil depth. It is called, simply, the surface effect. A whole class of flat water hydrofoil ferries have been built based on the surface effect.
It is also known that, in flat water, a hydrofoil can operate so as to remain on, or just slightly below the water surface. This mode of operation is called hydroplaning. It is characterized by the fact that the foil top surface is largely or completely exposed to the open air. There is conflicting evidence available on the effect of depth on lift, and hence, on the height stability of a hydroplaning foil. Certainly there is some range in which lift increases with depth, since hydroplaning is observed in practice. At greater depth, this situation may reverse. At best, the situation is not clearly understood. I have found two academic papers that mention the problem. One is work of Dobay on surface ventilation in the proceedings of the Hydrofoil Symposium held at the 1965 spring meeting of the Society of Naval Architects and Marine Engineers, and the other is by French et al. in the Proceedings of the Hydrofoils and Air Cushion Vehicles meeting, Washington, D.C., Sep. 17-18, 1962.
It is generally believed that for a foil operating at a given speed, angle of attack, and depth, it will develop far less lift if it is hydroplaning than if is fully submerged. Consequently, the expectation is frequently voiced that there should be an equilibrium depth where a foil can remain, suspended as it were, between hydroplaning and fully submerged. This logic is faulty. Hydrodynamics does not provide any basis for the expectation that there is a stable state between the two separate and distinct states. Which state prevails at any given moment depends on the previous history. In practice this often leads to very erratic behavior. Erratic behavior, of course, is precisely the opposite of what is wanted for control. On the positive side, since hydroplaning is unknown below very shallow depths, generally thought to be of the order of one chord at least the vertical amplitude of motion of a foil alternating between hydroplaning and fully submerged is limited.
The plunging phenomenon described earlier is a worsening of the erratic behavior discussed in the previous paragraph. Plunging involves, in addition to hydroplaning and full submerged, a third foil state in which the top surface is not exposed to the open air, but rather, to air enclosed in a compact volume, which I called a bubble. The new feature, the attached bubble, which can exist at considerable depth, greatly increases the possible vertical amplitude of motion of a foil that experiences all three states of surface wetting. A plunging foil is useless for control.
I have found one prior description of plunging. It is in U.S. Pat. No. 4,517,912 (1985) to Jones. Jones discloses a control means for hydrofoils for a sailing catamaran in which the attitude of a main foil is to be controlled by the depth of submersion of a smaller sensing foil, in consequence of which, the depth of the main foil, and hence the height of the craft itself, are kept constant. He notes in a single paragraph, that in chop, as the sensing foil approaches the surface, the water flow on its upper surface separates, decreasing the foil lift and causing it to dive. He then notes that what he calls the separated cavity tends to hang on to the foil, and that the foil continues to drop. He does not attempt to resolve the difficulty posed by the cavity.
U.S. Pat. No. 4,579,076 (1986) to Chaumette discloses a mechanism similar to Jones' for automatic height regulation of individual hydrofoil elements. In both devices, because of the short horizontal distance between the sensing foil and the foil it controls, control will tend to be abrupt. This abruptness will become especially acute in waves.
Jones states that his sensing foil should track at a small depth below the surface. He bases his analysis on the incorrect equilibrium depth expectation which I mentioned above.
Chaumette states that his sensing foil would track the surface itself, arguing simply that when the foil is in the air the lift would be negative, and when it is submerged it would be positive, and so in between there would be an equilibrium.
Because of plunging, neither Jones' nor Chaumette's sensing foil will stably track the surface.
Theoretical considerations associated with bubble attachment and shedding may be related to those associated with the high speed phenomenon of cavitation. I have found U.S. Pat. Nos. 3,946,688 (1976) to Gornstein, 4,949,919 (1990) to Wajnikonis, and 5,022,337 (1991) to Caldwell that speak to ventilation and cavitation. In addition, there is a considerable academic literature on these phenomena. Calculations of cavitation resistant foil sections are given by Shen and Eppler in the Journal of Ship Research, Vol 23, No 3, Sep. 1979, pp 209-217, and Vol 25, No 3, Sep. 1981, pp 191-200. However, in none of this work have I found any discussion specifically about foils that do not form air bubbles or that shed them rapidly if they form.
I next describe prior art related to yaw and roll stability, and to steering.
U.S. Pat. No. 3,742,890 (1973) to Hubbard contains an excellent discussion of the theory of yaw stability and turning for hydrofoil craft. In the context of a small ship, the patent discloses a modification of a canard configured hydrofoil craft that leads to improved steering during takeoff and in waves. The improvement is effected by use of a rigidly joined canard and streamlined support assembly that pivots on a bearing at the junction of the support and the ship hull in such a way that the support is able to swivel freely into alignment with the incident water flow.
U.S. Pat. No. 3,804,048 (1974) to Cline discloses a method for roll stabilization, which in Cline's embodiment is the same physical device as disclosed in Hubbard.
Both these disclosures are silent on the possible effect that the free trailing of the canard assembly might have on ventilation, either of the canard foil, or of the support.
U.S. Pat. No. 3,999,496 (1976) to Mirande discloses another modification of the standard canard assembly, in which the canard foil and support are, as is usual, rigidly attached to the watercraft hull, but where the support is surrounded by a streamlined fairing mounted on bearings so that the fairing can rotate around the support. Mirande's disclosure pertains to large hydrofoil ships, and the rotating fairing is specifically meant to be power actuated as a means of steering. The disclosure contains a critique of the prior steering art, including Hubbard's device, in which Mirande concludes that force requirements of known steering methods would be too great for application to large ships. Mirande makes no mention of the possibility that the fairing be left free to trail.
U.S. Pat. No. 3,421,468 (1969) to Newsom discloses a pedal powered watercraft that includes a sort of bow rudder that apparently comprises a fairing that rotates on a support. Here, too, no mention is made of possible benefit from free trailing.
Finally, I turn to the prior hydrofoil sailboard art itself. Of the four designs listed previously, only one, Coulter et al, deals adequately with the issue of height control or addresses the problems associated with ventilation, and that design has drawbacks that severely degrade performance.
Coulter et al. use a tandem pair of surface piercing hydrofoils, which control height automatically as a function of speed. These surface piercing foils also provide automatic roll stability. However, surface piercing foils in general have a number of disadvantages compared to fully submerged lifting foils: wave-making resistance where the foils pierce the surface is significant; ventilation of the lifting portions of the foils by air flow from the surface along the diagonal foil members must be blocked, usually by fences, which add drag; the same well known hydrodynamic considerations that lead to the height control and roll stability also lead to a rough ride in waves. This last is a significant problem in the context of a hydrofoil sailboard, since one of the great potential performance advantages, a very smooth ride in moderate sized waves, is lost at the outset. This advantage is realisable only with submerged, or submergible, foils. Coulter's design has another more specific problem, which becomes overwhelming in waves: if the tandem foils are mounted close together as illustrated in his disclosure, pitch stability becomes insufficient; if the foils are spread wider apart fore and aft, steering becomes impossible since each of the four surface piercing foil regions has a continually varying degree of immersion and consequent varying response to yaw.
The Harken Company offered a hydrofoil conversion kit for conventional windsurfers, but it did not catch on and was withdrawn from the market. This kit consisted of a main lifting foil and a pair of smaller, auxiliary lifting foils attached to the main foil by a fuselage mounted substantially parallel with the board. The main foil was attached to a streamlined strut that was inserted into the daggerboard slot of the windsurfer. There was no automatic height control at all, and successful operation required the sailor to make constant attitude adjustments to keep the foils properly submerged. When the sailor failed to do this, the foils alternately broached the surface into the air and crashed back into the water in a cyclical instability called porpoising. (Porpoising is a much less subtle instability than the plunging analysed previously, although both result in the inability of the sailor to control the craft. Porpoising behavior might well be complicated by a degree of plunging.) Avoiding porpoising turned out to be too great an effort for enjoyable long term use of the board.
The patent of Steinberg discloses an airplane foil configuration but implements no automatic height control, and so like the Harken design, is liable to porpoising. Steinberg's disclosure emphasises mechanical means for swiveling, pivoting, or hinging the foils to allow them to be positioned for attitude stability and to be retracted from the operational position.
In addition to ignoring height control, both the Harken design and Steinberg's take no precautions against the various difficulties associated with foil ventilation.
One embodiment of the disclosure of Jankowski is a canard design with the canard mounted on a thin rod and the main foil on a streamlined strut. The canard is about half the size of the main foil. The supports for main foil and canard are substantially the same length, and the foils are mounted on these supports at the same attack angle, which according to the disclosure is meant bring both main and canard to the surface at sufficiently high speed. At still higher speeds, Jankowski envisions rolling the board and foils to reduce wetted surface, and thus drag. This embodiment has some attractive features, but fails in the details necessary to make a stably workable craft.
Since the foils are supposed to fly on the water surface, the design would, of course, if it worked as described, provide height control.
A minor problem with the design as disclosed is that Jankowski's insistence on mounting the main and canard at the same attack will lead to attitude instability when both foils are submerged. This can easily be corrected by increasing the attack of the canard beyond that of the main.
More significantly, the design overlooks the problems of ventilation of foils operating at or near the water surface. The most obvious of these is that, before take off, when the canard is still submerged, the thin rod that supports it provides a perfect ventilation path, running along its trailing stagnation line, to the canard. The resulting ventilation of the top of the canard severely reduces its lift, and, since the main lifting foil with its streamlined support and absence of fences will suffer ventilation as a rather poorly determined function of yaw, the resulting system will suffer eratic losses and resumptions of pitch stability. If the foils are somehow brought to the surface, they will not stay there. Penetration of even a small wave will cause the canard to dive. Some means of blocking this ventilation path along the canard support is necessary.
As with all the prior hydrofoil sailboard designs, Jankowski does not recognize nor circumvent plunging.
In addition to these ventilation problems, which prevent steady operation of Jankowski's hydrofoil as disclosed, his design is liable to an entirely independent disadvantage stemming from the fact that both lifting foils are meant to operate on the water surface. Since hydrofoils are most efficient when flown substantially submerged, he must pay a considerable price in increased drag and consequent lessened speed relative to what would be possible if one or both foils were kept submerged. We shall discuss this issue further in subsequent sections of the disclosure.
The reason that Jankowski uses a thin rod to support the canard rather than the more obvious streamlined strut is unexplained in the disclosure. A good reason, however, is known in the prior art, and is discussed, for example, by Hubbard. It is that the rod provides relatively little lateral force when the board yaws, hence does not interfere with the normal steering control and yaw stabilization that comes with an aft location of the center of lateral resistance.
Jankowski clearly means to use the main and canard foils primarily for lift, and he discloses a daggerboard and skeg combination to balance lateral sail force. This leads to other problems since these vertical foils, exposed to the air after board takeoff, will ventilate. This must be prevented. His notion of rolling the board (presumably to windward) at higher speed would have the effect of unloading the originally vertical foils and transferring a portion of their lift to the main and canard. The daggerboard 12 shown in FIG. 1 of Jankowski becomes superfluous when the lateral resistance is provided by rolling the board.
Thus, all of the hydrofoil sailboard prior art suffers from one or more of the following deficiencies:
(a) poor pitch stability PA1 (b) lack of automatic height control; PA1 (c) poor steerability; PA1 (d) poor yaw stability; PA1 (e) poor roll stability; PA1 (f) succeptibility to foil ventilation; PA1 (g) severely degraded behavior in waves; PA1 (h) low foil efficiency.
Even in the more general hydrofoil prior art, the important topic of air bubble shedding and its relation to foil plunging is not addressed to any useful degree.